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Bu4NI Catalyzed Dehydrogenative Coupling of Diaryl Phosphinic Acids with Csp3-H Bonds of Arenes Bi-Quan Xiong, Gang Wang, Congshan Zhou, Yu Liu, Pan-Liang Zhang, and Ke-Wen Tang J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b02422 • Publication Date (Web): 25 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017
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Bu4NI Catalyzed Dehydrogenative Coupling of Diaryl Phosphinic Acids with Csp3-H Bonds of Arenes Biquan Xiong,* Gang Wang, Congshan Zhou, Yu Liu, Panliang Zhang, Kewen Tang* Department of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang, 414006, P.R.China.
[email protected];
[email protected]. RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) Abstract
An efficient phosphorylation of Csp3-H bonds of arenes with diaryl phosphinic acids via Bu4NI catalyzed dehydrogenative coupling has been developed. This transformation proceeds efficiently under transition metal-free reaction conditions, and represents a straightforward method to prepare valuable organophosphorus compounds from the readily available arenes and diaryl phosphinic acids.
As fundamental starting materials, organophosphorus compounds are versatile substrates in organic transformations. Especially some phosphate esters are very useful in functional materials and industrial science (e.g.,tris(2-ethylhexyl) phosphate is used as a plasticizer for vinyl polymers and triaryl phosphates are also manufactured commercially and used as additives for gasoline, polymer ACS Paragon Plus Environment
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plasticizers.).1-3 Phosphoric acid esters are of great commercial importance, and a valuable summary has been prepared by Toy and Walsh.4 The increasing interest of these compounds is mainly related to the wide presence of the phosphonic functionality (acid or ester) in many natural or synthetic bioactive compounds. Nonetheless, they are costly due to rare availability from nature. As depicted in Scheme 1, the most straightforward method of preparing such compounds is the straightforward transformation pathway between a P(O)-H/P(O)-X compounds and a nucleophiles, apart from the conventional nucleophilic substitution protocols.
Scheme 1 Traditional methods for the synthesis of phosphonate/phosphate esters.
The direct phosphorylation of nucleophiles is extensively used for the synthesis of organophosphorus compounds, but these approaches suffer from the low tolerance of functional groups and substrate limitations (e.g., Atherton-Todd reaction, nucleophilic substitutions).5a In 2013, Prabhu et al. reported a green, direct cross-coupling of phosphites with alcohols in the presence of I2/H2O2 at room temperature.5b In 2016, Chen and Han et al. further reported an efficient procedure based on ironcatalyzed dehydrogenative coupling of P(O)-H compounds with alcohols, using Fe(AcAc)2 as iron source and toluene as solvent, under N2 atmosphere.5c Nolan et al. disclosed the direct synthesis of phosphorus esters by transesterification mediated by N-heterocyclic carbenes in 2005.5d Later, Tang and Zhao et al. investigated the phosphorylation of benzyl C-H bonds via a cross-dehydrogenative coupling ACS Paragon Plus Environment
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path of using P(O)-H compounds as the starting materials.5e Although there are a large number of studies on the phosphorylation of nucleophiles, the use of P(O)-OH compounds as starting materials is rare. Indeed, literatures about the metal-catalyzed cross-coupling reaction of P(O)-OH compounds with Csp3-H bonds of arenes have not been reported. Thus, to develop an efficient and convenient method for the selective functionalization of P(O)-OH compounds is highly desired in organophosphorus chemistry. To avoid the prefunctionalization of reactants, the direct dehydrogenative coupling of Csp3-H bonds of arenes with organophosphorus compounds containing a P(O)-OH moiety under mild conditions will become an efficient and promising strategy for the formation of phosphinic and phosphoric esters.6-7 Very rencently, we have reported the direct esterification of P(O)-OH compounds with alcohols or phenols under mild reaction conditions.8 As an ongoing effort on the activation of P(O)-OH compounds, we herein report an efficient and simple oxidative dehydrogenative coupling of P(O)-OH compounds with Csp3-H bonds of arenes under mild conditions using a cheap quaternary ammonium salt catalyst. Compared with P(O)-H or P-Cl compounds, P(O)-OH compounds are more air- and/or moisture-stable, and the use of them is more cost-saving and environment-benign.9-11 The reaction of diphenyl phosphinic acid (1a) with toluene (2a) with the addition of TBHP (tert-butyl hydroperoxide, 2.0 equivalent), Bu4NI (tetrabutylammonium iodide, 10 mol%) and 1.0 mL of toluene under air gave the desired product of benzyl diphenyl phosphonate (3a) in 67% yield. In the reaction, toluene was in excess and served as solvent. Then we concentrated on the optimization of reaction conditions. At the initial, we screened several catalysts for the reaction, and found that Bu4NI was the best. To our surprise, Bu4NBr, 18-crown-6-ether and I2 showed negative to the reaction (Table 1, entries1-4). Different oxidants such as K2S2O8, TBPB (tert-butyl peroxybenzoate), TBHP (tert-butyl hydroperoxide), H2O2, m-CPBA (3-chloroperoxybenzoic), H3K5O18S4 and DTPB (di-tert-butyl peroxide) were further tested for the reaction. It was worth noting that the organic oxidants were much more effective than the inorganic oxidants. This phenomenon may be ascribe to the reaction of inorganic bases with P(O)-OH compounds to form the corresponding salts in the reaction, which inhibited the cross-coupling process (Table 1, entries 5-10). With the addition of TBHP increased to 3.0 equivalents, ACS Paragon Plus Environment
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the yield of 3a was increased from 67% to 76%. Further increasing the addition of TBHP did not result in the raise for the yield of 3a significantly, so we adopted the 3.0 equivalents of TBHP as the oxidant for the reaction. The catalyst loading also affected the reaction obviously. With the amount of Bu4NI increased from 20 mol% to 30 mol%, the expected coupling product of 3a was gained in 83% and 85% yields, so we chose 20 mol% of Bu4NI as the best catalyst loading (Table 1, entries 11-14). When the reaction was operated in 60 oC, it was only 47% yield of 3a generated after the reaction. Further increasing the temperature from 80 oC to 100 oC, the yield of 3a was decreased from 83% to 61%. In order to expand the applications of the reaction, different solvents were investigated (CH2Cl2, Dioxane, DMF, ClCH2CH2Cl, CH3CN, CH3OH, THF), and CH2Cl2 was found to be the best, affording 3a in 98% yield (Table 1, entries 17-23). Therefore, the optimal reaction conditions are as follows: diphenyl phosphinic acid (0.5 mmol), toluene (0.5 mmol), TBHP (1.5 mmol), Bu4NI (20 mol%), CH2Cl2 (0.5 mL), 80 oC, 12 h.
Table1 Optimization of the reaction conditions a).
Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Cat. (mol%) Bu4NI (10) Bu4NBr (10) 18-crown-6-ether (10) I2 (10) Bu4NI (10) Bu4NI (10) Bu4NI (10) Bu4NI (10) Bu4NI (10) Bu4NI (10) Bu4NI (10) Bu4NI (10) Bu4NI (20) Bu4NI (30) Bu4NI (20) Bu4NI (20) Bu4NI (20)
Oxidant (equiv) TBHP (2.0) TBHP (2.0) TBHP (2.0) TBHP (2.0) K2S2O8 (2.0) TBPB (2.0) H2O2 (2.0) m-CPBA (2.0) H3K5O18S4 (2.0) DTBP (2.0) TBHP (3.0) TBHP (4.0) TBHP (3.0) TBHP (3.0) TBHP (3.0) TBHP (3.0) TBHP (3.0)
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Solvent Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene CH2Cl2
Yield b) 67% 0% 0% 0% 26% 36% trace trace 0% 0% 76% 78% 83% 85% 47% c) 61% d) 98%
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a
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18 Bu4NI (20) TBHP (3.0) Dioxane 0% 19 Bu4NI (20) TBHP (3.0) DMF 0% 20 Bu4NI (20) TBHP (3.0) DCE
